The present invention relates generally to wet flue gas desulfurization (FGD) scrubbers and in particular to systems employing bleed streams or ex situ forced oxidation to oxidize sulfites to sulfates.
Typical wet FGD scrubbers, sometimes referred to as absorbers, consist of two major components: the scrubbing zone in which the actual gas scrubbing takes place and a reaction tank to allow efficient utilization of the reagent. The liquid reagent sprayed in the scrubbing zone captures sulfur dioxide (SO2) forming sulfites and bisulfites. These systems run free of scale if the oxidation of sulfites to sulfates is kept below about 15% (inhibited oxidation) or above 98% (forced oxidation). One means of controlling scale formation in an FGD system is to force oxidization of the sulfites to sulfates by bubbling air through the recirculated reagent.
Many present-day wet FGD scrubbers are single loop forced oxidation systems in which the scrubbing zone and the reaction tank are combined into one structure within the wet scrubber in what is referred to as in situ forced oxidation. A known single loop, in situ forced oxidation wet FGD scrubber 100 is shown in FIG. 1. Flue gas enters the scrubber at an inlet 12 located above the internal reaction tank and passes through a scrubbing zone consisting of a series of spray header levels 14 having a plurality of nozzles 16 which spray liquid reagent recirculated from the internal reaction tank by pumps 18. In the internal reaction tank, air is introduced to promote oxidation of the sulfites to sulfates. Other reactions such as reagent dissolution also occur. Sulfur oxides, produced in significant quantity by the combustion of coal, fuel oil or other fossil fuels, are removed by the liquid spray from the flue gas before the flue gas is exhausted to a stack (not shown) through an outlet 2. The liquid reagent is usually an alkaline slurry of lime, limestone, alkaline fly ash with supplemental lime, magnesium-promoted lime or a solution of sodium carbonate. The liquid reagent sprayed in the scrubbing zone captures SO2, forming sulfites and bisulfites. The pH of the partially reacted liquid reagent leaving the scrubbing zone falls to as low as about pH 4.5 depending on the reagent, stoichiometry, SO2 concentration and other design parameters. The low pH scrubbing liquid then falls into the reaction tank. Fresh liquid reagent is added to bring the pH of the liquid reagent in the tank back up to a preset level, for example from 5.8 to 6.2.
To maintain the reaction tank free of scale, the sulfites are oxidized to sulfates. This oxidation is typically accomplished by forcing air to a header 22 from a pump (not shown) which is distributed to a series of perforated sparger pipes 24 located in the reaction tank to allow air to be bubbled therein to force oxidation of the sulfites to sulfates in the reaction tank.
Older systems were designed to oxidize sulfites by bubbling the air through the reagent in a separate, external reaction tank. The formed sulfates were separated and disposed of. These systems were referred to as ex situ forced oxidation systems. Other systems bled a slip stream of reagent from the internal reaction tank, bubbled air through the reagent to oxidize the sulfites, and then returned the reagent back to the reaction tank, in an arrangement intermediate between the ex situ and in situ oxidation systems.
In some ex situ oxidation systems, the partially reacted liquid reagent is captured in the internal reaction tank, where fresh alkaline reagent is added to replace the reacted reagent and readjust the pH. A first stream is removed from the internal reaction tank and recirculated to the scrubbing zone. A second stream is removed from the internal reaction tank and sent to a separate external reaction tank, sometimes referred to as an oxidizer or oxidation tank. The preferred pH of the reagent in the oxidation process in the external reaction tank is 5 or lower. The pH of the readjusted reagent removed from the internal reaction tank, however, is about 5.8 and may be higher, due to the addition of fresh alkaline reagent. Therefore sulfuric acid is added to the oxidation stream or the separate external reaction tank to neutralize the alkali, and adjust the pH to the range preferred for promoting the oxidation process.
In some other older systems, a semi-in situ forced oxidation process was used in which a scoop collected nearly all of the sprayed liquid reagent, and sent it to an external oxidation tank. The contents of the tank were then pumped directly back to the FGD tower, rather than to a de-watering system. Only a bleed stream containing fresh alkali was removed from the process stream for dewatering.
Yet another FGD system employed a bowl in a double-loop operation to collect all of the liquid reagent from the absorber stage of the FGD system, however no attempt was made to minimize the use of sulfuric acid.
In a double-loop system, there are two loops, which are virtually separate from each other. The scrubbing loop contains fresh alkaline liquid reagent. The liquid reagent is then sprayed over several layers of packing to enhance the SO2 removal capability of the system. The contact between the reagent liquid on the packing and the flue gas causes the pH of the liquid reagent leaving the packing to drop, similar to the single-loop system. Then, the partially reacted liquid reagent, which is fairly low in pH, is collected in a bowl and sent to an external reaction or oxidation tank.
In a double-loop system, fresh liquid reagent is added to the external oxidation tank to maintain the process set point pH, and the adjusted pH liquid reagent is recirculated from the oxidation tank to the spray zone over the packing. Liquid reagent from the external oxidation tank overflows into the bottom of the scrubber, under the bowl, and is recirculated through a second loop to a set of headers, also located under the bowl. The function of these headers is to humidify the flue gas entering the scrubber so that wet/dry interface deposits do not form and assist in the scrubbing process. Partially reacted reagent liquid is usually drawn from the bottom of the scrubber based on a preset level. The lower loop usually runs at a lower pH than the upper loop to improve limestone utilization and reduce operating costs.
FIG. 2 illustrates a known double-loop flue gas desulfurization system comprising a housing, generally designated 5, having an inlet 12 near the bottom of the housing 5 for incoming flue gas. An outlet 2 is located at the top of housing 5 for the exit of flue gas after undergoing a scrubbing process within the housing 5.
Fresh liquid reagent is fed into feed tank 10 and pumped by pumps 30 to a plurality of upper level spray headers 20 located near the top of the housing 5 through a feed line 32 which is connected to the feed tank 10, the pumps 30 and the upper level spray headers 20. The liquid reagent sprayed by the upper level spray headers 20 has a high pH and is sprayed onto a packing 40, which is usually layered and has a depth of 2 to 3 feet, to enhance absorption of the SO2 in the high pH liquid reagent and filter exiting flue gas before the flue gas is channeled through outlet 2.
The sprayed liquid reagent from the upper level spray headers 20 trickles over and through the packing 40, and is diverted by a shroud ring 50 which is disposed concentrically around an inner diameter of the housing 5 for channeling liquid reagent through its inner opening to a bowl 60 located directly beneath the shroud ring 50 which collects the liquid reagent. The shroud ring 50 and the bowl 60 prevent the upper loop liquid reagent from being channeled into the internal reaction tank 70 located at the bottom of the housing 5. The liquid reagent collected in bowl 60 is directed back into the feed tank 10 by a return line 62 which is connected to the bowl 60 and the feed tank 10.
As the level of liquid reagent rises in feed tank 10, excess or overflow liquid reagent overflows back into the housing 5 to the internal reaction tank 70 through an overflow line 80 which is connected to the feed tank 10 and the housing 5 at the internal reaction tank 70. Liquid reagent from the internal reaction tank 70 is pumped by quencher pumps 71 to an array of lower level spray headers 90 through a lower feed line 72 which is connected to the internal reaction tank 70. The pumps 71 and the lower level spray headers 90 are used for quenching and scrubbing the flue gas, and improving the overall reagent utilization within the scrubber housing 5. The level of liquid reagent in internal reaction tank 70 is maintained below the inlet 12 through the use of purge means, i.e. a purge stream, indicated at 7 which is well-known in the art.